Auto: mirroring installation structure in build tree.

This makes the build tree more organized, which is good for adding new
stuff.  Now, it's useful for example for adding manual pages in man3/,
but it may be useful in the future for example for extending the build
system to run linters (e.g., clang-tidy(1), Clang analyzer, ...) on the
C source code.

Previously, the build tree was quite flat, and looked like this (after
`./configure && make`):

    $ tree -I src build
    build
    ├── Makefile
    ├── autoconf.data
    ├── autoconf.err
    ├── echo
    ├── libnxt.a
    ├── nxt_auto_config.h
    ├── nxt_version.h
    ├── unitd
    └── unitd.8

    1 directory, 9 files

And after this patch, it looks like this:

    $ tree -I src build
    build
    ├── Makefile
    ├── autoconf.data
    ├── autoconf.err
    ├── bin
    │   └── echo
    ├── include
    │   ├── nxt_auto_config.h
    │   └── nxt_version.h
    ├── lib
    │   ├── libnxt.a
    │   └── unit
    │       └── modules
    ├── sbin
    │   └── unitd
    ├── share
    │   └── man
    │       └── man8
    │           └── unitd.8
    └── var
        ├── lib
        │   └── unit
        ├── log
        │   └── unit
        └── run
            └── unit

    17 directories, 9 files

It also solves one issue introduced in
5a37171f73 ("Added default values for pathnames.").  Before that
commit, it was possible to run unitd from the build system
(`./build/unitd`).  Now, since it expects files in a very specific
location, that has been broken.  By having a directory structure that
mirrors the installation, it's possible to trick it to believe it's
installed, and run it from there:

    $ ./configure --prefix=./build
    $ make
    $ ./build/sbin/unitd

Fixes: 5a37171f73 ("Added default values for pathnames.")
Reported-by: Liam Crilly <liam@nginx.com>
Reviewed-by: Konstantin Pavlov <thresh@nginx.com>
Reviewed-by: Andrew Clayton <a.clayton@nginx.com>
Cc: Andrei Zeliankou <zelenkov@nginx.com>
Cc: Zhidao Hong <z.hong@f5.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

test/conftest.py

@@ -350,8 +350,11 @@ def unit_run(state_dir=None):
     if not option.restart and 'unitd' in unit_instance:
         return unit_instance
 
-    build_dir = f'{option.current_dir}/build'
-    unitd = f'{build_dir}/unitd'
+    builddir   = f'{option.current_dir}/build'
+    libdir     = f'{builddir}/lib'
+    modulesdir = f'{libdir}/unit/modules'
+    sbindir    = f'{builddir}/sbin'
+    unitd      = f'{sbindir}/unitd'
 
     if not os.path.isfile(unitd):
         exit('Could not find unit')
@@ -359,12 +362,12 @@ def unit_run(state_dir=None):
     temp_dir = tempfile.mkdtemp(prefix='unit-test-')
     public_dir(temp_dir)
 
-    if oct(stat.S_IMODE(os.stat(build_dir).st_mode)) != '0o777':
-        public_dir(build_dir)
+    if oct(stat.S_IMODE(os.stat(builddir).st_mode)) != '0o777':
+        public_dir(builddir)
 
-    state = f'{temp_dir}/state' if state_dir is None else state_dir
-    if not os.path.isdir(state):
-        os.mkdir(state)
+    statedir = f'{temp_dir}/state' if state_dir is None else state_dir
+    if not os.path.isdir(statedir):
+        os.mkdir(statedir)
 
     control_sock = f'{temp_dir}/control.unit.sock'
 
@@ -372,9 +375,9 @@ def unit_run(state_dir=None):
         unitd,
         '--no-daemon',
         '--modulesdir',
-        build_dir,
+        modulesdir,
         '--statedir',
-        state,
+        statedir,
         '--pid',
         f'{temp_dir}/unit.pid',
         '--log',